Field of the Invention
In general, the present invention pertains to a class of mechanical power transmissions having two alternately usable parallel transmitting paths between a single input and a single output so arranged that a change in the direction of rotation of the input changes the path through which power is transmitted.
More specifically, the present invention pertains to a related subclass of pedal and crank driven chain and sprocket geared bicycle transmissions so arranged that reversal of the rotational direction of pedaling inherently selects an alternate transmitting path resolved at the bicycle rear hub to accomplish a higher or lower forward driving speed ratio. Implicit in such a self-contained all mechanical arrangement is:                1. A first forward pedaling mode transmitting path input/output must be co-rotational and incorporate a first forward driving oriented overrunning clutch, and        2. A second backward pedaling mode transmitting path input/output must be counter-rotational and incorporate a second forward driving oriented overrunning clutch.Then when a given pedaling mode is input the associated overrunning clutch engages to transmit a forward driving effort via its modal transmitting path while the un-associated overrunning clutch overruns (i.e., turns freely) to disengage its modal transmitting path, and when the rate of either pedaling mode input is less than the output (coasting) both overrunning clutches overrun to disengage both modal transmitting paths.        
Description of the Related Art
In the early part of the 20th Century, both the Hirondelle and Terrot companies in France manufactured bicycles equipped with two-speed transmissions of the above described subclass which inherently shifted to a higher forward driving speed ratio when the rider pedaled backward. Both of these competing transmission systems were marketed to the public as “Retro-Direct,” as translated from the French. See for instance U.S. Pat. No. 0,797,814 to Mimard (manufactured by Hirondelle) and FR 338127 to Terrot & Company.
Of the same era and over the ensuing years, differently configured Retro-Direct bicycle transmissions were conceived and patented. See for instance FR 354891 to Terrot & Company, U.S. Pat. No. 0,954,672 to Didierjean, U.S. Pat. No. 5,254,042 to Chung-Suk and U.S. Pat. No. 6,695,332 B1 to Kang.
Summarizing the principal attributes for each of the above cited patents:
U.S. Pat. No. 0,797,814 to Mimard. The alternate forward driving speed ratio transmitting paths are implemented by reversibly recirculating a drive sprocket driven endless chain from a first rear hub located forward pedaling mode lower speed ratio driven sprocket via an idler sprocket mounted to and below the rider-right chain stay frame member to a second rear hub located backward pedaling mode higher speed ratio driven sprocket, and back. A forward driving overrunning clutch interfaces each counter-rotating driven sprocket to the rear hub. Comments: The idler sprocket location avoids chain path crossing interference, but requires the chain to re-align to each of the driven sprockets over a short distance, exacerbating chain to driven sprockets mis-alignment. Retro-Direct “do-it-yourself” enthusiasts tend to copy this design.
FR patent 338127 to Terrot & Company. The alternate forward driving speed ratio transmitting paths are implemented by a direct first forward pedaling mode lower speed ratio endless chain drive and a second backward pedaling mode higher speed ratio endless chain drive reversibly interfaced to the first chain drive via an intermediate sprocket cluster pair take-off. A forward driving overrunning clutch interfaces each counter-rotating driven sprocket to the rear hub. Comments: The physically parallel first and second chain drives facilitate chain to sprocket alignment. But the additional structure required to mount the sprocket cluster pair transfers additional forward and backward pedaling mode reaction loads to the bicycle frame.
FR patent 354891 to Terrot & Company. The alternate forward driving speed ratio transmitting paths are implemented by reversibly recirculating an endless chain from a first coplanar forward pedaling mode lower speed ratio drive and driven sprocket set to a second coplanar backward pedaling mode higher speed ratio drive and driven sprocket set, and back. The crank located drive sprockets are interfaced to the crank shaft via oppositely oriented overrunning clutches such that each pedaling mode action is exclusively transmitted to its mode effective segment of the chain. In addition, the rear hub located driven sprockets are also interfaced to the rear hub via overrunning clutches, but with each overrunning clutch similarly oriented to transmit a forward driving effort to the rear hub. Comments: This configuration employs more overrunning clutches than needed to achieve alternate transmitting path switching. Either the crank located or the hub located sprockets could be rigidly connected to their respective input or output members provided such coaxial and rigidly connected sprockets are of the same pitch diameter, thereby assuring the recirculated chain circulates both the crank and rear hub axes at the same link count rate. Also, at normal bicycle speed ratios, overrunning clutches located at the crank axis must transmit more torque than if located at the rear hub.
U.S. Pat. No. 0,954,672 to Didierjean. The alternate forward driving speed ratio transmitting paths are implemented by reversibly recirculating an endless chain from a first coplanar forward pedaling mode drive and driven sprocket set to a second coplanar backward pedaling mode drive and driven sprocket set, and back. The crank located drive sprockets are interfaced to the crank shaft via oppositely oriented overrunning clutches such that each pedaling mode action is exclusively transmitted to its mode effective segment of the chain. Both driven sprockets are of the same pitch diameter and rigidly connected to the rear hub as a unit. The relationship between an alternate forward driving speed ratio and a pedaling mode is specified as optional. Comments: At normal bicycle speed ratios, overrunning clutches located at the crank axis must transmit more torque than if located at the rear hub.
U.S. Pat. No. 5,254,042 to Chung-Suk, preferred embodiment. The alternate forward driving speed ratio transmitting paths are implemented by a conventional first forward pedaling mode endless chain drive and a second backward pedaling mode endless chain drive with its input/output rotation reversed via a symmetrical pair of idler sprockets mounted to the rear of the bicycle frame in the vicinity of the rear hub. A forward driving overrunning clutch interfaces each counter-rotating driven sprocket to the rear hub. Also optionally incorporated into the second backward pedaling mode chain drive is a manually operated multiple driven sprocket selecting chain derailleur system to add additional backward pedaling mode speed ratios. Comments: The separate parallel first and second chain drives facilitate chain to sprocket alignment. But the additional structure required to mount the backward pedaling mode idler sprocket pair transfers an additional backward pedaling mode reaction load to the bicycle frame.
U.S. Pat. No. 6,695,332 B1 to Kang, preferred embodiment. The alternate forward driving speed ratio transmitting paths are implemented by an indirect idler sprocket routed first backward pedaling mode endless chain drive and a second forward pedaling mode endless chain drive reversibly interfaced to the first chain drive via an intermediate sprocket cluster pair take-off. A forward driving overrunning clutch interfaces each counter-rotating driven sprocket to the rear hub. An additional manually operated clutch disconnects the rear hub from this transmission to permit the rider to roll the bicycle backward (simple Retro-Direct transmissions lock-up when back driven). Comments: The separate parallel first and second chain drives facilitate chain to sprocket alignment. But the additional structure required to mount the sprocket cluster pair and idler sprocket add additional forward and backward pedaling mode reaction loads to the bicycle frame. Also as presented, this transmission features equal forward and backward pedaling speed ratios, but its kinematics does not preclude unequal speed ratios.
Non-patent references Terrot & Co. Season 1904 Product Catalog, 2nd ed., 9 & 30 and Berto, Frank, The Dancing Chain, 4th ed., Cycle Publishing, 2012, 67-68 further explain and illustrate the Retro-Direct bicycle transmission concept and history.
Non-patent reference Wilson, David, Bicycling Science, 3rd ed., MIT Press, 2004, 86-88 gives the results of actual backward pedaling tests which show the efficacy of backward pedaling as nearly equal, or in some cases even superior, to forward pedaling.
Concurrently with and since the advent of the Retro-Direct bicycle transmission, other multi-speed transmission technologies have evolved to dominate the bicycle marketplace. Principally among these are multi-speed geared rear hubs and multiple sprocket selecting endless chain derailleur systems. Geared hubs now provide up to 14 progressive speed ratios. Derailleur systems can now reposition an endless chain between as many as 11 hub located driven sprockets and 3 crank located drive sprockets in any combination (but certain combinations in the extreme are not practical for “cross-chaining” reasons).
Also evolved among a certain set of cyclists (especially those who are urban dwelling commuters or bicycling purists) is a desire to return to bicycles of less complexity and/or minimalist aesthetics, with the side benefit of less required maintenance, i.e., bicycles equipped with a simple single speed endless chain transmission. However, single speed bicycles are hard to accelerate from a dead stop (intersection safety issue) and bog down on moderate hills (utility issue). But, what if an automatic second speed could be added with little additional complexity or loss of efficiency to help with starting out and/or moderate inclines?